The present invention relates to water jet propulsion units for use in water borne craft
This specification describes three water jet propulsion unit designs which contain a pair of counter-rotating impellers in in-line arrangement being driven forwardly on two coaxially arranged shafts. The means for driving the impellers are typically described in our NZ Patent No. 256488.
The designs depart from previous design and operating criteria, in that we require that the downstream impeller, in each case, operate at atmospheric pressure. Unlike the designs described in our NZ patent No 256488, where a hydraulic balance is maintained so that nozzle/internal pump pressures are in the range of about 0 to 276 kPa, in these designs only the upstream impeller/nozzle section operates within this pressure regime. Notwithstanding this, the upstream impeller/nozzle section may be also configured to operate at pressures above 276 kPA.
In energy terms this means that the downstream impeller blades, in these new designs, impart kinetic energy directly to the jet stream. Further advantages include the removal of back pressure effects on the downstream impeller and losses arising from pressure energy conversion at the nozzle outlet. In these designs the nozzle is now placed between the impellers and the opening, downstream of the downstream impeller, is now merely an outlet for the pump (As opposed to being a nozzle). The introduction of air to the downstream impeller. FIGS. 2, 3, 4, 5 and 6 further reduces frictional losses in the impeller casing but more importantly allows the downstream impeller to operate at atmospheric pressure.
To facilitate priming of the upstream impeller 5 an external pressure control priming device 10, comprising a collapsible skirt and peripheral spring, as seen in FIGS. 1, 2, 5 and 6 is placed between the two impellers 5 and 6. In FIG. 3 the pressure control priming device 19 is fixed to the centre of the upstream impeller 5 and consists of a collapsible skirt 20 within which is placed a plunger cone 21 and tensioning spring (not shown) whereby the pressure of the water forces the skirt 20 and plunger cone 21 in and out. The air inlet(s) 12 in the pump casing, FIGS. 2, 3, 4, 5 and 6 are provided with close-off flaps 13, FIG. 2 which are pressure controlled. Once a primed condition is achieved they remain open to permit continuous air entry. The provision of air inlets 12 thus allows the downstream impeller 6 to assist in priming the pump when they are closed. In FIG. 1 no air entry is permitted between the two impellers, but delivery rates between the two impellers must be carefully adjusted to ensure that the two impellers are hydraulically balanced in respect of flow rate, so that the downstream impeller always operates at atmospheric pressure.
A further improvement allows for the blades of the downstream impeller 6 to be automatically adjusted whereby the peripheral blade angles of the impeller 6 may be varied or calibrated according to the helical flow impinging on it from the upstream impeller 5. This feature is made possible because the blade to pump housing clearances are much greater than that required of a pressure pump so that the blades of the impeller 6 may be rotated slightly within the circular casing of the pump housing 8.
In very simple terms, the devices described are thus a pressurised pump section, containing the upstream impeller, followed by a propeller operating at atmospheric pressure, enclosed in a casing.
In a further design departure, not shown, the downstream section of the unit may simply consist of a ringed impeller whereby a ring is fixed directly to the outer edge of the impeller blades. No pump casing thus being required.
Also not described is a pump of essentially the same design configuration and having the same operating criteria, as described in any of the drawings, whereby the upstream impeller is of xe2x80x9cmixed flow designxe2x80x9d, followed by a downstream impeller of xe2x80x9caxial flowxe2x80x9d design. In this case the pressure control priming device is also between the impellers, together with the features already outlined for the totally axial flow design (FIGS. 1 to 6).
The designs are based on the principle of a high mass, low pressure and throttled configuration as described in our NZ patent 256488, such that improved efficiency is achieved by maximising the flow rate through the jet propulsion unit at the lowest possible internal unit pressure. Typically, impeller peripheral blade angles fall in the range of about 30 to 50 degrees, depending on power input but may fall outside this range should impeller diameters be altered or the pumps operating requirements change. Impeller peripheral tip speeds, relative to in-pump flow velocities, are usually limited to the range of about 45 to 65 meters/second, to restrict the damaging effects of cavitation. For specific applications, for example boat racing, where high boat speed is required, such a peripheral tip speed restriction may, however, be ignored by the user. The provision of air to the downstream impeller also helps to reduce the effects of cavitation. In respect of impeller design, the downstream impeller is no longer required to have a xe2x80x9cpressurexe2x80x9d configuration where the blades are normally aligned or over-lapped. Instead the blades may have a more open architecture as applies in conventional propeller design or a xe2x80x9ccleaverxe2x80x9d shape typical of those found in surface piercing drives. In our case however it is desirable to maintain the outer edge of the blade so that a large portion is contiguous with the wall of the pump housing in order to better control the amount of work carried out by the blade.